Combined field and impact ionization source for mass spectrometers

ABSTRACT

A combined field and impact ionization source includes a conducting housing embracing the trajectory of an electron beam for producing ions by impact. The conducting housing includes an ion exit gap through which an electric field leads ions produced by impact to a mass spectrometer. The conducting housing also functions as a counterelectrode that coacts with an emitter electrode of very small radius of curvature to comprise a field ionization source when the conducting housing is highly negative, such as 10 kilovolts, relative to the emitter electrode to produce ions by field ionization that are guided through the ion exit gap to the mass spectrometer.

United States Patent Inventors Curt Brunnee;

Kurt Elfenbein, both of Bremen, Germany Appl. No. 681,256 Filed Nov. 7, 1967 Patented June 1, I971 Assignee Varian Mat G.m.b. H.

Bremen, Germany Priority Nov. 19, 1966 Germany B89894 COMBINED FIELD AND IMPACT IONIZATION SOURCE FOR MASS SPECTROMETERS 7 Claims, 3 Drawing Figs.

US. Cl 250/413, 313/63, 313/230 Int. Cl H0lj 39/34 Field of Search 250/419 G,

[56] References Cited UNITED STATES PATENTS 3,115,591 12/1963 Brunnee 313/231 3,274,436 9/1966 Reich 3 l 3/63X 3,313,934 4/1967 Beckey 250/419 3,405,263 10/1968 Wanless et a1. 250/419 Primary Examiner-William F. Lindquist Attorney-Wolf, Greenfield and Sacks ABSTRACT: A combined field and impact ionization source includes a conducting housing embracing the trajectory of an electron beam for producing ions by impact. The conducting housing includes an ion exit gap through which an electric field leads ions produced by impact to a mass spectrometer. The conducting housing also functions as a counterelectrode that coacts with an emitter electrode of very small radius of curvature to comprise a field ionization source when the conducting housing is highly negative, such as 10 kilovolts, relative to the emitter electrode to produce ions by field ionization that are guided through the ion exit gap to the mass spectrometer.

INVENTORS cuRT BRUNNEE KURT ELFENBEHV I 'mrgg gq gg AT-TGR NEKS COMBINED FIELD AND IMPACT IONIZATION SOURCE FOR MASS SPECTROMETEIRS BACKGROUND OF THE INVENTION Known combined field ion and electron impact ion sources have the disadvantage that for the impact ionization and the field ionization, separate spatially separated ionization zones .are present. With the ionization of gas samples, which are admitted to both ionization zones through an inlet pipe, this is not of importance. However, when a vaporization oven is provided, for vaporization of nongaseous samples, this has to be differently positioned when changing from field ion to electron impact operation, in order to bring one or the other of the ionization zones into the vapor beam region ofthe oven.

SUMMARY OF THE INVENTION The problem underlying the invention is to provide a combined field ion and electron impact ion source which enables ionization according to both procedures, of easily and also of difficultly vaporizable substances with the substance supply not having to be repositioned. This problem can be solved according to the invention in that the emitter electrode of the field ion source is arranged in the impact chamber of the impact ion source and that the same exit gap with its metallic boundary wall serves as a boundary for the impact chamber when operating as an impact ion source and is switched to serve as the counterelectrode relative to the emitter electrode when operating as a field ion source, it being thereby achieved that the ionization zones for both kinds of ionization are spatially coincident or overlapping, and that for investigating difficultly vaporizable materials, the vaporization oven for these materials has to be provided only at one place, the expense for seals for the oven being thereby considerably lower. Moreover, one and the same specimen can be measured according to the one and the other procedure, during the vaporization procedure, simply by performing electrical switching operations outside the vacuum.

A particularly advantageous construction for the ion source is obtained if the counterelectrode is provided in known manner with a collar surrounding the emitter electrode, and has a window for entry of the electrons into the impact chamber when impact ionization is used.

In order to avoid impairment of the emitter electrode during the impact ionization, due to impurities, it is advantageous to arrange the emitter electrode so that it can be withdrawn from the impact chamber. For this purpose, the emitter electrode can be carried by a member having only a small surface, e.g. a swordlike body, which can be inserted with the electrode through an opening in the wall of the impact chamber housing opposite the outlet gap of the chamber, and also withdrawn.

The problem underlying the invention can also be solved in that in an arrangement having spatially separated ionization zones at the impact chamber, a vaporization oven is connected in such a way that the ionization zone for the impact ionization and also the ionization zone for the field ionization lies in the vapor beam region of the vaporization oven. Preferably, the central beam direction of vaporization oven lies between the two ionization zones.

In order to make the invention clearly understood, reference will now to made to the accompanying drawings which are given by way of example and in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic view of an embodiment of an ion source according to the invention;

FIG. la is a partial section along the line 11-" of FIG. 1, and

FIG. 2 is a diagrammatic view ofa second embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In the illustrated ion sources, a device for impact ionization is combined with a device for field ionization.

Impact ionization and field ionization take place in the ion source according to FIG. 1 and 2, both in a common ionization chamber 1. The impact ionization takes place by impacts of rapid electrons, which are emitted in the usual way by a glowing cathode 2 lying e.g. at +2900 v., and are accelerated by means ofa potential of v. between the cathode and the impact chamber, and pass through this on a trajectory 3. After passing through the impact chamber 1, the electrons are intercepted by an interceptor 4. The impact chamber 1 is surrounded by a metallic housing so as to form a defined electric field with low field gradients. This housing is in the form of a box of essentially rectangular cross section, the longitudinal axis of which coincides with the trajectory 3 of the electrons. At the level of the trajectory 3, windows 5 and 6 are provided for passage of the electrons. For the exit of the ions produced in the ion impact chamber, an exit gap 8 is provided in a longitudinal wall 7, hereafter called the front wall, in front of which, insulated from the box, a lens 9 is provided for extracting the ions from the box and for accelerating the ions in the direction 10 to the separation tube 11 of the mass spectrometer. Between the lens 9 and the separation tube 11, one or more diaphragms 12 are provided for accelerating and focusmg.

The box surrounding the impact chamber 1 consists of two parts, a cap-shaped part 14 with a rear wall 13 which borders the impact chamber 1 on five sides, and the sixth wall 7 which is separated and insulated therefrom, and which forms the front wall in which the exit slot 8 is provided. The cap shaped part 14, designated box in the following, is at a potential of +3000 v., the acceleration voltage of 100 v. being established between the cathode 2 and the impact chamber 1, for the electrons. The front wall 7 of the box is connected with a changeover switch 15 by which the front wall can be connected to a potential of +2995 v. for the operation as an impact ionization source, or for operation as a field ionization source can be connected to a potential of 9 kv.

With operation as an impact ion source, the front wall 7 with its exit gap 8 serves by virtue of its small negative potential relative to the box 14 for reinforcing the lens 9, in order to move the ions formed by the electron impact in the box 1, in the direction 10 towards the separating tube 11. It should be noted that for impact ionization operation, the front wall 7 could be mechanically and electrically connected with the remainder of the box 14, since the penetration of the lens field -through the exit gap 8 can suffice to withdraw the ions from the impact chamber.

With operation as a field ion source, the front wall 7 with the exit gap 8 serves as a counter electrode to the emitter electrode 16 and as a withdrawal diaphragm for the field ions for moving these field ions in the direction 10 to the separation tube 11 of the mass spectrometer.

The emitter electrode 16 is disposed in the chamber 1 on the side of the trajectory 3 opposite the exit gap 8, for the impact ionization, so that with both kinds of ionization essentially the same ionization zone 17 results, which is shown by cross shading in the drawing. The emitter electrode can consist of a very thin platinum wire arranged parallel to the trajectory 3 or to the exit gap 8, which is maintained at the same potential of +3000 v. as the box 14. With operation as a field ion source, a potential of l2 kv. exists between the emitter electrode 16 and the counterelectrode 17. Thus, in the region of the emitter electrode, as a result of its small radius of curvature, such a high field strength is achieved that in the vicinity of the emitter electrode, field ions are formed due to the socalled tunnel effect.

For improving the field distribution and the operation of the withdrawal diaphragm, the counterelectrode 7 is provided with a collar 18 which extends around the emitter electrode 16, and which is provided in the vicinity of the trajectory 3 with windows 19, 20 for passage of the electrons when operating as an impact ion source.

The emitter electrode 16 is provided on a sword-shaped carrier 21 which extends from the outside through a slot 22 in the rear wall of the box 14 and which is carried by a lever 24 rockable about an axis 23. By means of a traction cable 38, which may be sealed relative to the housing, e.g. by a bellows, the lever arm 24 can be swung in order to withdraw the emitter electrode 16 out of the impact chamber 1 when operating the ion source as a pulse ion source, so as to avoid a gradual contamination of the emitter electrode 16 by electrons impinging thereon.

In FIG. 2, the second embodiment of the ion source according to the invention is illustrated. In this case, the emitter electrode 16, as in the previously known combined impact ion and field ionization sources is disposed outside the impact chamber 1 for the impact ionization, and the counterelectrode 26 for the field ionization lies with its ion exit slot 27 in or in front of an opening 28 of the rear wall 13 of the impact chamber box 14.

This combined ion source thus has spatially separated ionization zones 29 and 30. In order, in spite of this spatial separation of the ionization zones, also to achieve guidance to both of the ionization zones of substances vaporized by an oven 25 without changing the position of the oven, when analyzing difficultly vaporizable substances, the oven 25 is so arranged that the ionization zone 29 for the field ionization and also the ionization zone 30 for the impact ionization lie in the vapor beam region 31 of the vaporization oven. For this purpose, the oven is connected to the underside ofthe impact chamber box 14 in such a manner that the vapor beam passes through the central region of the electron trajectory 3 for the impact ionization and through the exit slot 27 of the counterelectrode 26 in the vicinity of the emitter electrode 16, for the field ionization.

Many modifications and other embodiments are possible within the scope of the invention. Thus, the front wall 7 with the remainder of the box 14 could be maintained at the same potential of 3 kv. when operating as an impact ion source and also when operating as a field ion source. In this case, the emitter electrode would have to be placed at a correspondingly higher potential of for example +12 kv. The illustrated ion sources are suitable for the analysis of various substances.

I claim:

1. A combined field and impact ionization source comprising,

an electrically conducting housing having input and output apertures with an intermediate aperture therebetween,

a source adjacent said input aperture of an electron beam having a trajectory through said input and output apertures,

an emitter electrode adjacent to said intermediate aperture for coaction with said conducting housing functioning as a counterelectrode comprising a field ionization source,

means defining an exit gap for passing ions produced by impact and field ionization to a mass spectrometer,

means applying predetermined potentials to said conducting housing including the portion thereof surrounding said intermediate aperture for selectively establishing an impact ionization electric field for both moving electrons along said trajectory to produce ions by impact and move the ions thus produced through said exit gap to a mass spectrometer,

and means applying predetermined potential to said emitter electrode and said conducting housing including the por tion thereof surrounding said intermediate aperture for selectively establishing a field ionization electric field for producing ions by field ionization and moving the ions thus produced through said exit gap to a mass spectrome ter.

2. A combined field and impact ionization source in accordance with claim 1 and further comprising means for relatively positioning said conducting housing, said electron beam source and said emitter electrode so that the regions of impact ionization and of field ionization substantially coincide.

3. A combined field and impact ionization source in accordance withclaim l and further comprising means for relatlvely positioning said conducting housing, said electron beam source and said emitter electrode so that the region of impact ionization is along said trajectory, and said emitter electrode and the region of field ionization are outside said conducting housing.

4. A combined field and impact ionization source in accordance with claim 3 and further comprising a source of a vapor beam that embraces both said ionization regions.

5. A combined field and impact ionization source in accordance with claim I wherein said conducting housing is formed with a collar surrounding at least a portion of the region between said housing and said emitter electrode.

6. A combined field and impact ionization source in accordance with claim 1 and further comprising means for selectively withdrawing said emitter electrode away from said conducting housing.

7. A combined field and impact ionization source in accordance with claim 6 wherein said source includes an enclosure and said means for selectively withdrawing comprises a pivotable lever attached to said emitter electrode and secured to said enclosure about a pivot point, with means extending through said enclosure for operating said lever. 

1. A combined field and impact ionization source comprising, an electrically conducting housing having input and output apertures with an intermediate aperture therebetween, a source adjacent said input aperture of an electron beam having a trajectory through said input and output apertures, an emitter electrode adjacent to said intermediate aperTure for coaction with said conducting housing functioning as a counterelectrode comprising a field ionization source, means defining an exit gap for passing ions produced by impact and field ionization to a mass spectrometer, means applying predetermined potentials to said conducting housing including the portion thereof surrounding said intermediate aperture for selectively establishing an impact ionization electric field for both moving electrons along said trajectory to produce ions by impact and move the ions thus produced through said exit gap to a mass spectrometer, and means applying predetermined potential to said emitter electrode and said conducting housing including the portion thereof surrounding said intermediate aperture for selectively establishing a field ionization electric field for producing ions by field ionization and moving the ions thus produced through said exit gap to a mass spectrometer.
 2. A combined field and impact ionization source in accordance with claim 1 and further comprising means for relatively positioning said conducting housing, said electron beam source and said emitter electrode so that the regions of impact ionization and of field ionization substantially coincide.
 3. A combined field and impact ionization source in accordance with claim 1 and further comprising means for relatively positioning said conducting housing, said electron beam source and said emitter electrode so that the region of impact ionization is along said trajectory, and said emitter electrode and the region of field ionization are outside said conducting housing.
 4. A combined field and impact ionization source in accordance with claim 3 and further comprising a source of a vapor beam that embraces both said ionization regions.
 5. A combined field and impact ionization source in accordance with claim 1 wherein said conducting housing is formed with a collar surrounding at least a portion of the region between said housing and said emitter electrode.
 6. A combined field and impact ionization source in accordance with claim 1 and further comprising means for selectively withdrawing said emitter electrode away from said conducting housing.
 7. A combined field and impact ionization source in accordance with claim 6 wherein said source includes an enclosure and said means for selectively withdrawing comprises a pivotable lever attached to said emitter electrode and secured to said enclosure about a pivot point, with means extending through said enclosure for operating said lever. 